From: Robert Dober <robert.dober@...>
Date: 2007-02-26T19:13:43+09:00
Subject: Re: extract a random number of items from an array

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On 2/25/07, Mingquan Chen <chen.mingquan@gmail.com> wrote:
>
> If you use this way, then how we you get exactly n values randomly from
> the
> given array?
>
> I doubt the correct of this way.
>
> My solution should meet your requirement. :-)
>
> On 2/26/07, Josselin <josselin@wanadoo.fr> wrote:
> >
> > On 2007-02-25 17:41:58 +0100, James Edward Gray II
> > <james@grayproductions.net> said:
> >
> > > On Feb 25, 2007, at 10:20 AM, Josselin wrote:
> > >
> > >> given an array of values, how should I extract an random number of
> > >> these values
> > >>
> > >> a = [13, 15, 17, 18, 19, 20, 21, 23, 24, 25, 26, 27, 28, 29, 30,  31,
> > >> 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56,  57,
> > >> 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73,  74,
> > >> 75, 76, 77, 78, 79, 81, 82, 83, 84, 85, 86, 88, 89, 90, 91, 92,  93,
> > >> 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106]
> > >
> > > Just give a random chance that a value is added to the result set:
> > >
> > >  >> a.select { rand(2).zero? }
> > > => [13, 15, 23, 24, 25, 26, 27, 28, 29, 31, 41, 42, 44, 45, 46, 47,
> > > 49, 51, 52, 53, 55, 58, 60, 61, 62, 66, 67, 68, 69, 70, 71, 72, 73,
> > > 75, 76, 78, 81, 82, 83, 89, 90, 91, 94, 95, 98, 99, 100, 101, 103,
> > > 104, 105, 106]
> > >  >> a.select { rand(2).zero? }
> > > => [13, 18, 19, 20, 21, 24, 26, 27, 28, 29, 30, 31, 41, 42, 44, 48,
> > > 49, 51, 52, 53, 55, 57, 58, 59, 62, 63, 64, 66, 68, 70, 79, 81, 82,
> > > 83, 86, 89, 90, 91, 92, 94, 97, 98, 102, 104, 105, 106]
> > >  >> a.select { rand(2).zero? }
> > > => [15, 17, 18, 19, 20, 21, 23, 25, 30, 41, 44, 45, 46, 49, 50, 51,
> > > 52, 53, 54, 57, 59, 63, 64, 66, 68, 70, 72, 73, 76, 78, 79, 81, 82,
> > > 90, 91, 95, 97, 101, 102, 104]
> > >
> > > Hope that helps.
> > >
> > > James Edward Gray II
> >
> > thanks James that's exactly what I need
> > I made various tests like     a.select { rand(10).zero? }
> > and I got different results..  as I want
> >
> > I try to understand the logic behind your solution (not to dry
> stupid...)
> >
> > select from array a  ...  {  for each item.  0#no 1#yes }   ok for
> > rand(2)  , if I use rand(10) less chances to get it
> > is that right ?
> >
> >
> >
>
Actually James' solution does not give you n elements, maybe you want to try
this
an_ary.sort_by{ rand }[0..n-1]  # Might use a second or two of CPU for
millions of elements though ;)
should do the trick unless you want a *stable* subarray.
Stable means that if x occurs on the LHS of y in an_ary it also does in your
subarray, but if I read you correctly you did not need this.

Cheers
Robert



-- 
We have not succeeded in answering all of our questions.
In fact, in some ways, we are more confused than ever.
But we feel we are confused on a higher level and about more important
things.
-Anonymous

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